Fundamental Neutron Physics

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

This project is devoted to research by the University of Michigan Fundamental Neutron Physics Group on two fundamental-neutron-physics measurements: 1) measurement of the neutron lifetime with precision of approximately one second using a cold-neutron beam at the NIST Center for Neutron Research (NCNR) and 2) measurement of the beta-neutrino correlation with the Nab/abBA/PANDA spectrometer at the Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory (ORNL). These are two of the five highest priority experiments identified in recommendations by a recent NSAC review of the field. The physics has broad impact across the fields of nuclear and particle physics and astrophysics. Both efforts enjoy strong collaborations and the support of the community as well as the scrutiny of critical reviews. The University of Michigan group has key technical and leadership roles in both efforts. At NIST, we will perform an absolute calibration of the neutron-flux monitor used in the proton-trap measurement of the neutron lifetime. The proton-trap apparatus was used the most recent beam-lifetime measurement with the result for the lifetime of (886.3 +/- 1.2 (stat) +/- 3.2(sys)) seconds, where the systematic error is dominated by uncertainties related to the neutron-flux monitor calibration. The flux-monitor calibration campaign includes two independent calibrations using a monochromatic neutron beam at NIST and thick targets of Boron-10 or liquid Helium-3. For the Boron-10 approach, alpha particles and gamma rays are counted, and a set of measurements has recently been completed that demonstrated internal consistency and precision that would reduce the systematic error to about 0.5 sec. For the Helium-3 approach, the heat generated by the charged particles in the 3He(n,p)3H reaction is measured calorimetrically and converted to a neutron flux. The calorimeter approach, originally developed by our group, has been shown to be capable of measuring the flux to 0.1 % with 10 hours of integration. A number of improvements specifically related to the challenges of the liquid Helium-3 target have been completed at the University of Michigan, and the device will be moved to NIST with the aim of completing the calibration campaign by 2013 and undertaking the new proton-trap lifetime measurement on the new NIST NGC beam line in 2014. The Nab spectrometer and experiment is to measure the beta-neutrino correlation at SNS. The magnetic-field-expansion spectrometer is designed to measure proton-electron coincidences using proton time-of-flight spectroscopy to extract the a parameter. It may also be possible to extract the Fierz interference b from electron spectroscopy in silicon detectors. The envisioned continuing physics program with the Nab spectrometer and polarized neutrons (abBA/PANDA) is extremely rich with the ability to over constrain V-A parameters for neutron decay and thus probe physics beyond the Standard Model. The University of Michigan group is responsible for all aspects of neutron polarization and polarimetry beginning, for Nab, by measuring the neutron polarization of the unpolarized FP-13 beam with sensitivity to the polarization of less than 0.01%. The neutron is a sub-atomic particle that comprises more than half of the matter in the world. Within the nuclei of most atoms, the neutron remains stable, but when freed from the nucleus, it is unstable. Free neutrons are an important tool for study of sub-atomic physics, because the decay and interactions of free neutrons reveal the interactions of its constituents and decay products. Neutrons also have spin, the quantum mechanical property of the most fundamental pieces of matter that distinguishes two states called spin-up and spin-down. Spin is responsible for the nuclear magnetism exploited, for example, in NMR and MRI. The spin states also affect the decay and interactions of neutrons, and so the control of neutron spin becomes useful for more detailed study of sub-atomic interactions. This project envisions a program to incisively and carefully advance both the precision and the accuracy of the neutron lifetime and the correlation of electron and neutrino momenta in neutron decay. The neutron lifetime is a measured fundamental quantity that impacts physics from the formation of the elements to the solar energy cycle to the structure of the Standard-Model of the weak interaction. Discrepancies among recent measurements have actually led to reduced confidence in the value of the neutron lifetime, and a new measurement using different techniques is essential to resolving the discrepancies. The measurement of the neutron lifetime using a cold neutron beam will provide a different approach, and accurate measurement of the neutron flux is crucial to the experiment. The neutron flux will be measured with a detector that has sensitivity that scales in the same way as the detection of neutron decays, but which must be calibrated by an absolute method. Absolute calibration will be achieved by measuring the heat produced when the neutrons are absorbed by Helium-3 in a cryogenic target at two degrees above absolute zero. The goal of this calibration is to provide an accurate neutron-flux measurement and determine the neutron lifetime with one-second precision. The angle between the electron and neutrino produced in neutron decay is sensitive to the fundamental strength of the weak interaction when combined with the neutron lifetime. A broader class of measurements using polarized neutron can be used to probe beyond the Standard Model of particle physics. A new spectrometer designed to measure both the proton and electron energies in neutron decay will first measure the electron-neutrino correlation and then be used with polarized neutrons. This work probes deep intellectual questions about the most fundamental pieces of matter. The aim is to collect data that will help complete the picture of elementary particles and their interactions. The techniques have much broader impact. Laser polarized Helium-3 used to polarize neutrons has applications to materials science, and quantum information research. This project is a remarkable training ground for undergraduate and graduate students and post doctoral fellows. The technical challenges combined with the deep intellectual issues provide motivation and develop technical skills. Undergraduates will gain research experience working along with graduate and post doctoral fellows. Graduate students emerge broadly capable and move on to prepare for faculty or national lab positions as well as interdisciplinary research. This work has also led to development of new courses for non-physics majors, to a set of public lectures on Nuclear Magnets and Neutrinos and leadership in communicating science to the interested general population. In the context of probing fundamental problems of physics, exciting in its own right, the hardest problems produce the most innovative solutions with spin-offs unimaginable at the outset. Atomic clocks, enhanced MRI, and experiments that probe the origin of matter all follow from the control of nuclear magnetism and neutron spins.